In recent years, cloths dyed with an indigo dye, particularly a bleached denim comprising a blue denim having been subjected to bleaching processing, have become popular.
Bleaching of a blue denim includes a case where a blue denim is relatively uniformly bleached and a case where a blue denim is non-uniformly bleached. Usually, in the case where a blue denim is relatively uniformly bleached, dip bleaching is carried out by using sodium hypochlorite. In the case where a blue denim is non-uniformly bleached, bleaching is carried out by impregnating a pumice or the like with a sodium hypochlorite solution and, after drying, revolving and stirring a blue denim in a cleaning device. In any of these cases, sodium hypochlorite is mainly used as a bleaching agent. As other bleaching agents than sodium hypochlorite, U.S. Pat. No. 4,218,220 proposes use of trichloroisocyanuric acid as a bleaching agent.
In any of these methods, a step for removing chlorine by using a reducing agent such as sodium thiosulfate is introduced after the bleaching processing step.
The above-described case of using sodium hypochlorite as a bleaching agent involves such a defect that cotton fibers are deteriorated and, hence, when the degree of bleaching is further controlled, a method for controlling an available chlorine concentration of or processing time with a processing solution is employed. However, in this case, the bleaching power of sodium hypochlorite is so strong that it is not easy to control the degree of bleaching. Further, as a method for controlling the degree of bleaching, a method for changing a pH of or processing time with a processing solution could be considered, but this method is not desired because the pH of the sodium hypochlorite solution is lowered, or if the temperature of the processing solution is increased, a chlorine gas is volatilized.
In order to solve this problem, U.S. Pat. No. 4,218,220 proposes use of trichloroisocyanuric acid as a bleaching agent. However, trichloroisocyanuric acid is considerably acidic such that a 0.1% aqueous solution thereof has a pH of from 1 to 2, and its bleaching power is too high. Further, in the case that it is aimed to suppress the bleaching power of trichloroisocyanuric acid by adding an alkaline agent thereto, there are still such problems that not only is it dangerous because decomposition of trichloroisocyanuric acid causes generation of nitrogen chloride, but a blue denim is likely yellowed.
Bleaching of a blue denim is likely delicately influenced by not only the type of a bleaching agent used but the processing condition employed. Therefore, undesired phenomena such as deterioration of fibers by reduction in the tear strength and yellowing of the blue denim sometimes occur.